No, an extension’s username cannot be changed after it has been set.

As an alternative, the account admin can delete the existing extension, and then create a new one with the desired username.   Custom settings, such as voicemail greeting, additional Find Me List entries, Call Screening, etc. will need to be reestablished for the new extension.

IMPORTANT NOTES about deleting extensions:

• Deleting an extension will permanently remove it from the Aptela service, and remove all accessibility to its call history, including voicemail and faxes.  Be sure to save necessary information before pressing Delete.
• If you need to delete an extension that currently has a Menu option assigned to it, you will need to reassign the setting to another destination before pressing Delete.  Be sure to reassign the setting to the new extension after creating it.  
• If you delete an extension that currently has a phone number assigned directly to it, the phone number will automatically be rerouted to the main Call Tree after pressing Delete.  To have the phone number reassigned to the new extension, please submit an account change ticket and include the phone number and designated extension in the ticket description.